Online Tool Store
· 5 min read
How to Size a Trade From Your Risk Percentage
Manesh Jayawardhana
CIO & Co-founder
You’ve got a $20,000 trading account and you’ve decided you’re only willing to risk 1% of it on any single trade — that’s $200. You’ve picked an entry at $54.20 and set a stop-loss at $52.80, a $1.40 gap per share. Now the question is: how many shares does that actually let you buy? Eyeballing it and rounding to “about 140 shares” either leaves risk on the table or blows past your own rule, and neither is a small mistake when it happens trade after trade.
Position sizing is one of the few places in trading where the math isn’t optional or approximate — it’s the mechanism that keeps a string of losing trades from wiping out an account. Guessing at share count, or sizing based on “how much I feel like buying,” undermines the whole point of having a risk rule in the first place.
What position sizing actually involves
The core formula is: (account balance × risk percentage) ÷ (entry price − stop-loss price) = number of shares or units. The numerator is your dollar risk — the maximum you’re willing to lose on this one trade. The denominator is your per-share risk — how much you lose per unit if the stop-loss gets hit. Dividing one by the other tells you exactly how many shares keep your total dollar risk at or under your target.
This works the same way whether you’re trading stocks, forex lots, or futures contracts — the underlying logic (dollar risk ÷ per-unit risk = size) doesn’t change, only the units do.
Why people get this wrong
- Sizing based on how much capital they want to deploy, not how much they’re willing to lose. “I want to put $5,000 into this trade” ignores where the stop-loss actually sits.
- Widening the stop-loss to fit a desired share count. This backwards approach — deciding on size first, then adjusting the stop to match — defeats the purpose of risk-based sizing entirely.
- Rounding position size up “just to make it a round number.” Rounding 140.3 shares up to 150 for convenience quietly increases your actual risk above your intended percentage.
- Forgetting commissions and slippage. On thinly traded instruments or with high per-trade fees, the effective risk can run higher than the calculated stop-loss distance implies.
- Using the same risk percentage regardless of setup quality. Treating a high-conviction setup and a speculative one identically ignores that position size is a risk management tool, not a fixed ritual.
What a solid position size calculator looks like
Direct inputs for entry and stop-loss
You should enter the actual entry price and stop-loss price, not a pre-calculated “risk per share” number — letting the tool do that subtraction removes a common source of manual math errors.
A clear dollar-risk translation
Seeing “this is $200 of risk” alongside the share count keeps the actual dollar exposure visible, not buried behind a percentage that’s easy to lose track of mid-trade.
Support for different account sizes and risk percentages
Risk tolerance isn’t fixed — a calculator that lets you quickly compare 0.5% vs 1% vs 2% risk on the same setup shows how much the position size (and thus potential drawdown) shifts with each choice.
| Risk Level | Typical Use | Trade-off |
|---|---|---|
| 0.5% per trade | Conservative, high trade frequency | Smaller positions, slower account growth |
| 1% per trade | Common default for many traders | Balanced — a string of losses is survivable |
| 2%+ per trade | Higher conviction or fewer trades | Faster gains, but drawdowns compound quickly |
Common mistakes to avoid
- Deciding the position size first and fitting the stop-loss around it, instead of the other way around.
- Using a risk percentage that isn’t consistently applied trade to trade.
- Ignoring that a stop-loss can gap through on volatile instruments, meaning actual loss can exceed the calculated risk.
- Not recalculating position size as the account balance changes after wins or losses.
- Forgetting that position sizing manages risk per trade, not total portfolio risk across multiple open positions.
How to do it with Position Size Calculator
Online Tool Store’s Position Size Calculator runs entirely in your browser — no account details ever leave your device.
- Enter your account balance and the percentage you’re willing to risk on this trade.
- Enter your planned entry price and stop-loss price.
- See the exact share or unit count that keeps your dollar risk at your target percentage.
- Recalculate before every trade, since account balance changes after each win or loss.
Frequently asked questions
What risk percentage should I use per trade?
There’s no universal answer — many traders use somewhere between 0.5% and 2% per trade, with lower percentages favored by those trading more frequently or with less experience managing drawdowns.
Does position sizing protect against gap risk?
Not fully. A stop-loss order can be skipped over during a large overnight or news-driven gap, meaning actual loss can exceed the calculated risk even with correct position sizing.
How does this differ across stocks, forex, and futures?
The formula is the same, but the “per-unit risk” changes meaning — for stocks it’s dollars per share, for forex it’s typically pip value per lot, and for futures it’s the point value per contract. Make sure your entry and stop-loss inputs use consistent units for whichever instrument you’re sizing.
Final thought
Position sizing is the one part of a trading plan that doesn’t rely on being right about market direction — it just needs you to enter the real numbers and stick with the output.